Previous research has identified the motor neuron network that controls the muscle activity giving rise to the traveling body wave of the swimming medicinal leech, Hirudo medicinalis. The swimming rhythm has its origin in the interconnections of a small group of interneurons whose interactions not only generate the rhythm in each segmental ganglion, but also provide for the coordination of the rhythm along the entire animal. A mechanism for rhythm generation, recurrent cyclic inhibition, has been shown by theoretical analysis and modeling studies to account for the period and intra- and interganglionic phase relationships of the rhythmic activity of these interneurons. I propose to study the mechanisms generating the leech swimming movements in more detail. First, I plan to characterize the activity phases and interconnections of additional segmented oscillatory neurons. Second, I plan to identify and characterize interneurons that can function to initiate, terminate or otherwise modify the swimming rhythm. Several of these cells have already been identified. Third, I plan to continue to investigate the neuronal circuits that interconnect recently identified sensory structures at the leech body wall with the oscillatory interneurons. Finally, I plan to continue my studies on simulation of neuronal circuits with electronic analog neurons.